1. Field of the Invention
This invention relates to optical fibers with tapered tips, methods and systems that use such optical fibers, and methods of fabricating such optical fibers.
2. Discussion of the Related Art
Many near-field scanning optical microscopes (NSOMs) use an optical fiber to laterally scan the sample being imaged. The optical fiber has an end face that either collects image light from a region of the sample or delivers illumination light to a region of the sample. To increase lateral resolution, some NSOMs incorporate a scanning optical fiber whose tip has been gradually but substantially down-tapered. The down tapering produces an end face whose diameter is much smaller than the diameter of the untapered portion of the optical fiber. Decreasing the diameter of an end face has a tendency to reduce the lateral range in the sample from which light will be collected. For that reason, a NSOM whose scanning optical fiber has a down-tapered tip potentially has a higher lateral scanning resolution than another NSOM that is similar except for the down-tapering of the tip of the scanning optical fiber.
Down-tapering a tip of a scanning optical fiber increases light losses in the optical fiber for two reasons. First, the down-tapering produces a smaller end face, which is less efficient at collecting light than a larger end face. If the end face's diameter, d, is much smaller than the wavelength, λ, of the light collection efficiency is expected to scale with “d” as a positive power of d/λ, e.g., (d/λ)4. Second, down-tapering a single mode optical fiber typically causes light to leak out of the down-tapered portion of the optical fiber.
For both of the above-described reasons, optical fibers with down-tapered tips typically have lower light collection efficiencies than optical fibers that are similar except for the down-tapering. The lower light collection efficiency will lower the sensitivity of a NSOM whose scanning optical fiber has such a down-tapered tip. Thus, increasing a NSOM's lateral resolution by tapering-down the tip of the scanning optical fiber often involves a tradeoff of lower sensitivity for the NSOM.